Bacillus anthracis is an aerobic gram-positive spore-forming bacterium found naturally in wild and domesticated animals. Hanna, P. 1998. Curr. Top. Microbiol. Immunol. 225:13-35. It is highly resistant to environmental degradation, and it produces a tripartite toxin that reduces the ability of the host=s immune system to eliminate the pathogen. Id. Human exposure to anthrax typically arises following contact with infected livestock, and generally results in a mild form of cutaneous disease. Friedlander, A. M., and P. S. Brachman. 1998. Anthrax, p. 729-739. In S. A. Plotkin and E. A. Mortimer (ed.), Vaccines. W. B. Saunders, Philadelphia, Pa. Quinn, C. P. and P. C. Turnbull. 98 A.D. Anthrax, p. 799-818. In M. Ballow and M. Sussman (ed.), Topley and Wilson's Microbiology and Microbial Infections, Collier, London. However, anthrax spores designed for aerosol delivery were intentionally released by bioterrorists in the US in 2001. The resultant morbidity, mortality, and widespread panic underscored the potential for anthrax to be used as a bioterror agent as well as the need to improve the speed, magnitude and safety of anthrax vaccination. Lane, H. C., et al. Nat. Med. 7:1271-1273.
As noted in U.S. Patent Application 2004/0082530, the pathogenicity of B. anthracis is expressed in two ways: a toxic effect made evident by the appearance of an edema, and a so-called lethal toxic effect which may lead to death in infected individuals. There are two main virulence factors possessed by B. anthracis, a poly-D-glutamic capsule that inhibits phagocytosis and two binary toxins which are formed from combinations selected from three protein factors. These two binary toxins possess a common cell receptor-binding component which, when combined with either one of the other two factors forms an active toxin. The binding component present in both of the active toxins is non-toxic and is involved in the binding of the B. anthracis toxins to cell membranes in an infected host. The other two protein factors constitute the active elements responsible for the manifestation of either the toxic effect of the edema type or the toxic effect with lethal character. These two active factors are termed edematogenic factor (EF) and lethal factor (LF). The non-toxic factor responsible for binding to cell membranes is called protective antigen (PA) since, during immunization assays, the capacity to confer active protection against the disease was initially attributed to this factor. The three factors PA, LF and EF have been isolated and purified as reported by Fish et al. (1968) J. Bacteriol. 95:907-917, and the two toxins obtained by the combination of PA and LF and of PA and EF, have been characterized and described by Leppla et al. (1982) Proc. Natl. Acad. Sci. USA 79:3162-3166. The B. anthracis genes pag, cya and lef that encode the factors PA, EF and LF, respectively, are distributed on a plasmid termed “pX01” of B. anthracis, as described by Mikesell et al (1983) Infect. Immun. 39:371-376. In addition, the pag, cya and lef genes have been cloned and fully sequenced as described by Welkos et al. (1988) Gene 69:287-300; Escuyer et al. (1988) Gene 71:293-298; and Bragg et al. (1989) Gene 81:45-54.
Anthrax Vaccine Adsorbed (AVA) is the only anthrax vaccine licensed for human use in the U.S. It is prepared by adsorbing the culture filtrate of an attenuated toxinogenic non-encapsulated strain of B. anthracis (V770-NP1-R) onto aluminum hydroxide. Ivins, B. E., and S. L. Welkos. 1988. Eur. J. Epidemiol. 4:12-19. Studies show that protective antigen (PA), the core of anthrax toxin, is the major immunogen of AVA. Antibodies (Ab) against PA neutralize the toxin, inhibit spore germination, and improve the phagocytosis/killing of spores by macrophages. Ivins, B. E., et al. 1992. Infect. Immun. 60:662-668. Little, S. F., and B. E. Ivins. 1999. Microbes. Infect. 1:131-139. Welkos, S., et al. 2001. Microbiology 147:1677-1685. Welkos, S. L., and A. M. Friedlander. 1988. Microb. Pathog. 5:127-139. Vaccination with AVA requires a series of 6 immunizations delivered over 18 months followed by yearly boosters. Pittman, P. R., et al. 2001. Vaccine 20:972-978. Pittman, P. R., et al. 2002. Vaccine 20:1412-1420. This schedule has been linked to the development of adverse side effects including joint pain, gastrointestinal disorders, and pneumonia, leading many U.S. soldiers to refuse vaccination. Geier, D. A., and M. R. Geier. 2002. Clin. Exp. Rheumatol. 20:217-220. Ready, T. 2004. Nat. Med. 10:112. Strategies which reduce the dose and number of AVA immunizations required to achieve protection are expected to improve compliance.